Abstract: This study investigated the influence of different mass concentrations of catechin (0~1.00 mg/mL) on the physicochemical characteristics, structures, and gel properties of myosin, through analyses of the turbidity, particle size, Zeta potential, surface hydrophobicity, electrophoretic mobility, infrared spectra, fluorescence spectra, and gel quality of tilapia myosin. The results showed that myosin with added catechin generally exhibited lower turbidity and average particle size than myosin without added catechin, with these values rising with increasing catechin concentration. Zeta potential showed an overall decreasing trend from −2.048 to −5.05 mV. Surface hydrophobicity exhibited an initial decrease followed by an increase. At catechin doses of 0.25, 0.75, and 1.00 mg/mL, surface hydrophobicity was reduced by 23.56%, 44.71%, and 40.38%, respectively. Changes were most significant at a catechin concentration of 0.75 mg/mL, which led to an average particle size of 3275 nm, Zeta potential of −4.8 mV. Infrared and fluorescence spectra analyses revealed that myosin underwent changes in secondary and tertiary structure under the influence of catechin. Optimal gel properties were exhibited by tilapia myosin-catechin mixed gel at a catechin concentration of 0.75 mg/mL, with gel strength and water-retention capacity of the gel reaching 224.12 g·mm and 89%, respectively. Thus, catechin addition improved the physicochemical activities and altered the structures of tilapia myosin, enabling the enhancement of surimi gel properties.